1. Field of the Invention
This invention relates to computer systems and, more particularly to apparatus for providing masking which may be useful in extending a computer architecture from thirty-two to sixty-four bits and for other, more general, purposes.
2. History of the Prior Art
As computers have improved in speed and ability, there has been a constant demand for more addressable memory. The size of the address directly controls the size of memory which may be addressed. Each additional bit in an address doubles the amount of addressable memory. Thus, a system using sixty-four bit addresses provides two to the thirty-second power more addresses than does a system using thirty-two bit addresses. Consequently, researchers are today attempting to develop computer systems based on sixty-four bit architectures.
On the other hand, very powerful computer systems exist today which use thirty-two bit addresses. A very large amount of effective software exists for such systems for which substantial investments have already been made. These thirty-two bit systems could effectively handle larger and larger problems if there were an easy way to simply address more memory. Presuming that the only effective way to obtain more memory is to design systems using larger (e.g., sixty-four bit) addresses, it would be very foolish and economically disastrous to simply discard all of the effort presently invested in the design of thirty-two bit systems and their software. For this reason, it is an a priori requirement that any new computer system based on a new memory size for an existing architecture be able to use the old programs on the new architecture. A primary question in the design of such a system is, therefore, how to change memory address size and still be able to use the old programs on the new architecture.
A new system has been devised in which the hardware is able to handle both old thirty-two bit and new sixty-four bit programs using the same circuitry. This new system is simply able to run either old or new programs without any great amount of new hardware except essentially, that necessary to allow the larger addresses to be utilized and without any rewriting of old software to fit the new hardware. One portion of the new system is disclosed in U.S. patent application, Ser. No. 07/632,017, entitled METHOD AND APPARATUS FOR EXTENDING COMPUTER ARCHITECTURE FROM THIRTY-TWO TO SIXTY-FOUR BITS, Powell et al, filed on even date herewith and assigned to the assignee of this invention. The system utilizes an integer processor having registers designed to run programs of the largest one of the word sizes. Because of the use of two's complement arithmetic in such a processor, almost all arithmetic operations increase in size toward the most significant bit. Since the new register size provides thirty-two extra bits beyond the most significant bit used in thirty-two bit registers, almost all arithmetic operations for both formats may be carried out in the new register size without significant changes. For most instructions, a thirty-two bit program merely ignores the upper thirty-two bits. However, in certain operations carried on in thirty-two bit format, addresses are generated in the processor registers. The generation of these addresses may utilize more than the lower thirty-two bits of the sixty-four bit positions in the register. If these operations use more than a thirty-two bit address, then the operations will be carried out incorrectly. In order to assure that invalid information is not included when using a thirty-two bit address in a sixty-four bit register to load or store, the processor requires a method of masking the high order bits of the address.
Once an arrangement for masking high order bits of an address has been realized, it will be apparent to those skilled in the art that such an arrangement may find greater use than in allowing two different sizes of programs to be run in the same registers. For example, the ability to mask high order bits allows those bits to be used for a number of other purposes which are especially advantageous where a large address space is available yet may not find complete use.